Proton-conducting polymers are of great interest, in part due to their application in fuel cells. A serious problem with current polymers, including Nafion™, is a restricted temperature range limited to temperatures below around 80° C. Such membranes require water for proton conduction. However, higher temperature operation is valuable for a number of reasons, including 1) improved tolerance of the catalyst to carbon monoxide; 2) simplification of the cooling system; and 3) increasing proton conductivity at higher temperatures. Therefore, fuel cell operation at temperatures above 100° C., without the need for humidification, is highly desirable.
Proton carriers other than water have been used for high-temperature operation, such as phosphoric acid. Various polymer-phosphoric acid blends have been reported, including PEO-H3PO4, PVA-H3PO4, and H3PO4 doped in PEO-PMMA and polyacrylamine hydrogel. However, these materials suffer from various problems, including poor chemical stability due to hydrolysis of ether and amide groups, or low mechanical stability, particularly at high-temperature. Polybenzimidazole [PBI] blended with phosphoric acid is reported in U.S. Pat. No. 5,525,436. However, the acid quantity was quite high, reaching above 5 mols of H3PO4 per PBI repeat unit, and therefore there was a problem of leaching of free acid molecules from the membrane and consequent reduction of proton conductivity.
Hence, there is a need for improved polymer electrolyte membranes, particularly for operation at high temperatures.